SOHO Joint Observing Programme 75 TIME VARIATION AND DYNAMICS OF ACTIVE REGIONS LOOPS Authors: O. Kjeldseth-Moe (CDS), P. Brekke (CDS), J. Gurman (EIT), T. Tarbell (TRACE, MDI), O. Wikstol (CDS) Progress: First Version: December 16 1997 Objective: Investigate thoroughly rapid temporal variations observed in active region loops and relate them to the dynamical state of the loops. Conditions Necessary to Run: CDS can run this alone, but high cadence images from EIT and TRACE are essential whenever they can be obtained. For observations near the center of the solar disc, MDI support will add important information. Scientific Justification: CDS has observed very rapid time variations in loops above active regions at the solar limb in the temperature range 10 000 K to 1 MK (He I, O V, Mg IX). It is common that loops change their appearance appreciably over periods of 15-20 minutes and entire loops may even turn on and off in these short time intervals. Coronal loops change much more slowly and are usually not co-located with the transition region loops. The rapid changes in morphology is coupled to a strong dynamical state. Line shifts are seen corre- sponding to line-of-sight velocities of 50 km/s or more. Important objectives for this JOP include: - finding the shortest time time scales for the variations - observe with better spatial resolution in order to relate the phenomenon to fine structures in loops - obtain additional information on the variability at more temperatures Questions may be asked: Are the strong time variations of the loops related to other rapid time variations in the transition region such as the blinkers (JOP073)? How are the high velocities created? Is the observed behaviour related to extreme fine structure in loops? Whatever will be the answers to these and other questions it seems clear that the observed behaviour will require a complete change in our concept of the transition region and has profound implications for modelling and understan- ding of the solar atmosphere Pointing: Mainly this program is to be executed at the solar limb, but observations on the disc will also be made. Operating Details: CDS: Three sequences are run: O_LOOP1, O_LOOP2 and O_LOOP3. O_LOOP1 builds up a 160 x 240 arc second raster in 6 wavelength bands, i.e. He 1 584.33 A, O 3 599.66 A, O 5 629.73 A, Ne 6 562 A, Mg 9 368.06 A, and Fe 16 360.76 A, by stepping the 4 arc second slit in steps of 4 arc seconds. The exposure time at each position is 10 seconds giving a cadence of approximately 8-10 minutes. This is the ``work-horse'' for the CDS observations, and is run a number of times for a total duration of several hours. O_LOOP2 builds up a similar raster in 13 wavelength bands over a period of 45 minutes. Wavelengths include density sensitive lines and this raster is designed for a more thorough diagnostics. However, the sequence is not suitable for observing time variations since it obviously is not rapid enough. O_LOOP2 is therefore run only at the beginning or/and the end of the JOP. O_LOOP3 is a wide slit (90x150 arc second) movie sequ- ence of 60 exposures in three lines, He 1 584 A , O 5 630 A, and Mg 9 368 A. Exposure time is 6 seconds, giving a cadence of 8-12 seconds. This is CDS' most rapid sequence suitable for studies of structure changes, but spectral information is sacrificed. It is to be run once or a few times during the JOP and NEVER at the disc. The FOV should be placed outside the limb with only a small part in one corner touching the disc, preferably in a quiet area, to give a reference to the solar position. This is done to avoid flares with the wide slit open. TRACE: Rapid sequences in C 4 1550 A will be run at the limb. Off limb the entire emission in the wavelength band will be from the C 4 lines. Cadence will be of the order of 2 images per minute with a FOV of 320 x 320 arc seconds. Another option to be used would be images in 171 A (Fe 9-10). Like the CDS movie (O_LOOP3) these TRACE observations give a very high time resolution, but they do this with an angular resolution which is much higher than obtained with CDS or EIT. The obser- vations would thus reveal fine structure not to be seen in CDS or EIT. On the disc observing options with TRACE would include 1550 A, 1600 A, 1700 A and 171 A for a 2 minute time resolution. EIT: Rapid cadences will be done for part of the duration of the JOP. Images will include the 195 A band with a 2.5 minute cadence, running for up to 1.5 hour, and optionally the 304 A band with a similar cadence. The latter will be interesting for comparison with CDS rasters (O_LOOP1) and movies (O_LOOP3) in the 584 A He 1 line. He 1 304 A is also unique to EIT and is not observable with TRACE or CDS/NIS. Observations will, however, be limited owing to detector degradation for EIT. MDI: Magnetic observations in the high resolution field will be made for the less frequent runs of the JOP targeted on the disc.